/Engine/Source/Runtime/SlateCore/Public/Widgets/SWidget.h
e.g. Fullscreen games cannot summon a new window, so game SViewports will reply with EPopupMethod::UserCurrentWindow. This makes all the menu anchors within them use the current window.
virtual [FPopupMethodReply](API\Runtime\SlateCore\Input\FPopupMethodReply) OnQueryPopupMethod() const